Our invention relates to audio communication and more particularly, to arrangements for reducing reverberation and echo effects in audio systems.
In telephone and other audio communication systems, sound applied to an electroacoustic transducer from a single source often traverses a plurality of diverse paths between the source and the transducer. In addition to the direct path signal, delayed echo signals are obtained as a result of reflections from walls and other surfaces. The echoes are delayed with respect to the direct path signals and do not add in phase with the direct path signal. Consequently, the combination of direct path and echo signals causes distortion. If the position of the sound source is known, it is possible to place a transducer near the source so that inverse square law attenuation reduces the echo signals. Alternatively, a highly directional microphone aimed at the source can be used to enhance the direct path signal with respect to echo signals.
There are many systems, however, in which the direction of the sound source is variable or unpredictable. In conferencing arrangements, for example, a plurality of speakers in a room are served by a speakerphone set. The direction of sound is variable and the room reflections are generally not controlled. Consequently, adverse effects are distinctly noticeable and some electronic arrangement must be used to reduce echo and reverberation without changing room conditions.
One type prior art system for reducing multipath reverberative interference utilizes two or more spatially separated microphones, each receiving different versions of the same sound. The microphone outputs are directly combined so that reverberative effects are minimized. In another arrangement, the signals from a plurality of spatially separated microphones are processed to select the signal having least reverberative interference. These arrangements, however, require that one microphone be substantially closer to the sound source that the other microphones of the system. Other techniques use spectral analysis to select spectral portions of each of a plurality of microphone signals. The selected spectral portions are combined to produce a composite signal with reduced reverberation. The spectral techniques, however, employ relatively complex apparatus to partially reduce the echo effects.
A more direct solution to the reverberative interference problem is disclosed in U.S. Pat. No. 3,794,766 issued on Feb. 26, 1974 and assigned to the same assignee. In accordance with this patent, sound from a source is received by a pair of spatially separated microphones. Each microphone signal is passed through a delay and the delayed signals are cross-correlated in the time domain. The cross-correlation signal is used to control one delay, which delay is adjusted to maximize the cross-correlation signal. The delayed direct path signals are now aligned in phase, but the reverberation signals remain out of phase. The summing of the delayed signals produces an output signal with reduced reverberation.
The delayed microphone signals include complex direct path and echo signals. The echo signals are substantial replicas of the direct path signals and are therefore closely correlated with the direct path signals. Thus, the direct cross-correlation results in a composite of many peaks including peaks corresponding to delays between different echo signals and peaks corresponding to delays between echo signals and direct path signals as well as peaks for the direct path signals. Further, the correlations of speech signals do not generally produce sharp peaks. Unless the direct path signals are much stronger than the echo signals, the correlation signal which is a composite of many broad peaks may not be maximum when the direct path components of the delayed signals are coincident. Consequently, the reduction of reverberative effects is relatively poor without complex multiple cross-correlation arrangements.
It has been observed that the delay between microphone signals obtained from a single source can be better detected if the complex detailed waveforms of the delayed signals are changed by non-linear transformation. By transforming the delayed signals to reduce waveform detail, the direct path components are enhanced with respect to the echo components and the effect of signal similarity on the location of correlation peaks is reduced. It is therefore an object of the invention to provide an improved, simplified signal dereverberative arrangement which is not affected by the complex detailed nature of the audio signal.